The conventional Hall-Heroult process for aluminium production uses carbon anodes which are consumed by oxidation. The replacement of these consumable carbon anodes by substantially non-consumable anodes of ceramic oxide materials was suggested many years ago by Belyaev who investigated various sintered oxide materials including ferrites and demonstrated the feasibility of using these materials (Chem. Abstract 31 (1937) 8384 and 32 (1938) 6553). However, Belyaev's results with sintered ferrites, such as SnO.sub.2.Fe.sub.2 O.sub.3, NiO.Fe.sub.2 O.sub.3 and ZnO.Fe.sub.2 O.sub.3, show that the cathodic aluminium is contaminated with 4000-5000 ppm of tin, nickel or zinc and 12000-16000 ppm of iron, which rules out these materials for commercial use.
Considerable efforts have since been made to design expedients which offset the defects of the anode materials (see for example U.S. Pat. Nos. 3,974,046 and 4,057,480) and to develop new anode materials which stand up better to the operating conditions. Some of the main requirements of the ideal non-consumable anode material for aluminium production are: thermal stability and good electrical conductivity at the operating temperature (about 940.degree. C. to 1000.degree. C.); resistance to oxidation; little solubility in the melt; and non-contamination of the aluminium product with undesired impurities.
U.S. Pat. No. 4,039,401 discloses various stoichiometric sintered spinel oxides (excluding ferrites of the formula Me.sup.2+ Fe.sub.2.sup.3+ O.sub.4) but recognized that the spinels disclosed had poor conductivity, necessitating mixture thereof with various conductive perovskites or with other conductive agents in an amount of up to 50% of the material.
West German published patent application (Offenlegungsschrift) DE-OS No. 23 20 883 describes improvements over the known magnetite electrodes for aqueous electrolysis by providing a sintered material of the formula EQU M.sub.x Fe.sub.3-x O.sub.4
which can be rewritten ##STR1## where M represents Mn, Ni, Co, Mg, Cu, Zn and/or Cd and x is from 0.05 to 0.4. The data given show that when x is above 0.4 the conductivity of these materials drops dramatically and their use was therefore disconsidered.